Xerographic charging



1959 R. w. GUNDLACH 2,912,586

XEROGRAPHIC CHARGING ,v. Q a s L Filed Nov. 1, 1957 POWER SUPPLY FIG. 3

INVENTOR. Robert W. Gundlach ATTORNEY United States Patent XEROGRAPHICCILARGING Robert W. Gundlach, Spencerport, N.Y., assignor to HaloidXerox Inc., a corporation of New York Application November 1, 1957,Serial No. 693,866

6 Claims. (Cl. 250-49.5)

This invention relates in general to methods and means for selectivelydepositing electrostatic charge on insulating surfaces.

This invention is concerned with the application of a pattern of chargesto an insulating surface. Once deposited the charges may be used asthrough scanning or the like or they may be developed with chargedparticles, charged droplets, or the like.

It is therefore an object of this invention to provide new and improvedmeans of selectively depositing electrostatic charge on an insulatingsurface.

It is another object of this invention to provide new and improvedmethods of selectively depositing electrostatic charge on an insulatingsurface.

It is a further object of this invention to provide new and improvedelectromechanical methods and apparatus for forming an electrostaticcharge pattern on an insulating surface.

Other objects and advantages of the present invention will be morereadily apparent in view of the following detailed disclosure anddescription thereof, especially when read in conjunction with theaccompanying drawings, wherein:

Figure 1 is a schematic perspective view of one form of apparatusaccording to this invention;

Figure 2 is a schematic perspective view of another form of apparatusaccording to this invention; and

Figure 3 is a schematic perspective view of a third form of apparatusaccording to this invention.

For a better understanding of this invention reference is now had toFigure 1, wherein charging through selective deposition of electrostaticcharge on an insulating surface is shown. A chargeable member ispositioned on conductive backing plate 14. Conductive rollers 18 permitbacking plate 14 to move back and forth over conductive base plate 11and establish a conductive path between plate 11 and plate 14. Supportframes 12 are rigidly mounted to base plate 11 and act to support andposition charging cylinder 15 above base plate 11. Although only onesupport frame 12 is fully illustrated, both contain the same elements.Charging cylinder 15 is supported in frames 12 through axles 13 whichare journaled in electrical insulating blocks 16 which in turn areslidably mounted in frames 12. Charging cylinder 15 is biased againstchargeable member 10 by springs 17 pressing against insulating blocks16, whereby chargeable members 10 of varying thickness may beaccommodated. Charging cylinder 15 comprises an inner conductive core 20surrounded by an outer covering layer 21 of poor conductive material inwhich an insulating pattern 22 of insulating material is imbedded.Although preferably pattern 22 is imbedded in covering layer 21 as inthis embodiment, it may also extend outward above the surface ofcovering layer 21. A power supply 23 is connected to and suppliespotential to conductive core 20 of charging cylinder 15 with respect tobacking plate 14. Extending outward from one of the axle extensions 13is an electrically insulating crank handle 19.

The mechanism of this figure is operated by positioning a chargeablemember 10 on backing plate 14 with its leading edge against chargingcylinder 15. A potential difference is supplied from power supply 23between the conductive core '20 of charging cylinder 15 and blockingplate 14 through base plate 11. An operator may then turn handle 25which will cause the outer surface of charging cylinder 15 to rotateagainst a surface of chargeable member 10. As the chargeable member 10comes beneath charging cylinder 15 backing plate 14 rolls on rollers 18thereby allowing chargeable member 10 to move freely as motion isimparted to it by rotating charging cylinder 15. Springs 17 press onblocks 16 causing cylinder 15 to make pressure contact with chargeablemember 10. If initially an uncharged chargeable member is placed intothe mechanism as illustrated in Figure 1 and is moved beneath chargingcylinder 15, all areas of chargeable member 10 corresponding or adjacentto areas of poor conducting covering layer 21 of charging cylinder 15become charged following movement beneath charging cylinder 15. In areason the chargeable member which correspond with and contact areas ofinsulating image 22 of charging cylinder 15 no charge will deposit. Inthis figure a positive potential is supplied from power supply 23 toconductive core 20 with respect to backing plate 14 and the chargeswhich deposit on chargeable member 10, following movement of chargingcylinder 15 in the direction indicated, define an image pattern orelectrostatic charge pattern of no charge in areas of image and positivecharge in areas of background on the surface of the chargeable membercorresponding to the pattern on the covering layer 21 of chargingcylinder 15. If it is desirable to charge areas of backgroundnegatively, negative potential is supplied to conductive core 20 ofcharging cylinder 15 from power supply 23, and operation of themechanism is carried out as has already been described.

Reference is now had to Figure 2 wherein there is illustrated anotherembodiment of electrostatic charge image formation. In this embodimenttwo rollers 24 and 25 formed of conductive material, each with axles 28,are mounted in support frame 27. The lower roller 25 is journaled infixed insulating blocks 37; whereas the upper roller 24 is journaled inslidably mounted blocks 38 to allow roller movement upward and downwardto accommodate between the rollers material or layers of varyingthicknesses. Springs 34 bias blocks 38 and roller 24 downward. Althoughin this figure there is illustrated only one support frame 27 supportingand positioning the rollers 24 and 25, this has been done for purposesof clarity in the illustration and it is to be realized that a framesimilar to frame 27 is positioned on the other ends of the rollers.Motor 32 drives roller 25 through belt 31 and insulating pulley 30mounted on axle 28. When the lower roller 25 is rotating motion isimparted to the material between the rollers and the upper roller 24rotates freely as motion is imparted to it by moving material betweenthe rollers. Both rollers 24 and 25 are connected to power supply 33which supplies a positive potential to one roller and a negativepotential to the other. Desirably potential is supplied only whilematerial is between the rollers or the rollers are positioned at aslight distance from one another to prevent a short circuit brought onby contact made by the two conductive rollers while electricity is beingsupplied. In this embodiment the upper roller is supplied with negativepotential.

Passing between rollers 24 and 25 are chargeable member 10 and chargingmaster 35. Charging master 35 comprises a layer of poorly conductingmaterial 39 on which or in which there is formed insulating pattern 36.In this embodiment positive charge exists on the surface of chargeablemember prior to movement between the rollers. Movement between rollers24 and 25 causes negative charge fiow from upper roller 24 to thesurface of chargeable member 10 through the poor conducting areas 39 andno charge fiow through the insulating pattern areas 36 of the chargingmaster. The new charges reaching chargeable member 10 discharge thepre-existing positive charge except in areas of member 10 correspondingto pattern 36 where there is no effect. Thus, there is produced,following movement of a chargeable member 10 against a charging master35 through rollers 24 and 25, a chargeable member carrying charge onlyin areas corresponding to insulating pattern 36. It is to be realized ofcourse that the charge ble member may be charged negatively prior toins'e'r'ti betw een rollers 24 and 25, and are'aeaf charge may bedischarged by biasing the uppef' ro pditiii'fl'fbr by placing thesandwich or assembly b'tw fftlie rollers so that the charging mastercontacts the posit ixg roller. It is also to be realized that selectivein pattern configuration may also be carried out in connection with theembodiment illustrated in Figure 1 by precharging the chargeable member.Similarly, deposition of charge in background areas only, i.e., thosenot corresponding to insulating pattern 36, would be carried out in themechanism of Figure 2 if the chargeable member were not precharged priorto movement between the rollers. It is also to be realized that themechanism of this figure may be used to further charge rather thandischarge the background areas. Thus, for example, if a negativelyprecharged chargeable member were moved through rollers 24 and 25 withthe charging master 35 against the roller connected to the negative endof power supply 33, additional negative charge would flow through thepoor conducting areas of master 35 resulting in areas of differingnegative potential on the surfaceGFc'HaYEeEBIETrIcmber 10. Areas ofbackground in such an instance would have more negative charge thanareas corresponding to insulating pattern 36.

Although a motor drive is illustrated in connection with the apparatusof Figure 2, it is to be realized that the apparatus may be adapted formanual drive and may be tied in with other equipment. Similarly, theapparatus in Figure 1, which is shown to be manually driven, may bedriven by a motor or the like.

Reference is now had to Figure 3 wherein another embodiment of means andmethods of selective charge deposition is illustrated. A base plate 11of conductive material supports on its surface chargeable member 10.Hinged at hinges 42 to base plate 11 is pressure plate 43 comprisingconductive layer 44, poorly conductive layer 45, and handle 47. Handle47 is made of insulating material and is used to open and close pressureplate 43 away from and into contact with the upper surface of chargeablemember 10. Imbedded in poorly conductive layer 45 is a pattern ofinsulating material .46. Base plate 11 is connected to power supply 48as is conductive layer 44 of pressure plate 43. Power supply 48 suppliesa difference of potential to each of these members thereby creatingfields of force between these members. Optionally, a switch may bepositioned against the upper surface of base plate 11 to close the powercircuit when pressure plate 43 is closed against base plate 11.

In this embodiment, as in the previous embodiments, when potential isapplied between the conductive layer 44 of pressure plate 43 and baseplate 11 and when a chargeable member 10 is in position on base plate 11with the pressure plate 43 in closed position against chargeable member10 an electrostatic charge pattern corresponding to the insulatingpattern 46 will be formed on chargeable member 10. If negative potentialis applied to conductive layer 44 and base plate 11 is grounded orplaced at a positive potential and chargeable member 10 is notprecharged, then areas of background will be charged negatively; if apositive potential is applied to conductive layer 44 and support table11 is grounded or placed at a negative potential and chargeable member10 is not precharged, then background areas of chargeable member 10 willbe charged positively. If chargeable member 10 is precharged positivelyand a negative field generating potential is applied to conductive layer44, then areas of background Will be discharged and areas correspondingto insulating pattern 46 will remain charged on chargeable member 10.Similarly, if chargeable member 10 is precharged negatively and apositive field generating potential is applied to conductive layer 44areas of background will be discharged while other areas on chargeablemember 10 will remain negatively charged. If a precharge is given tochargeable member 10 which is of the same polarity as the potentialplaced on conductive layer 44, areas of background will have theprecharge strengthened while other areas will continue to hold the sameamount of charge. Thus, it is possible, following the techniquesdescribed and illustrated in connection with Figures 1, 2, and 3 of thisinvention and the modifications which will readily occur to thoseskilled in the art, to create on a chargeable member an electrostaticcharge pattern of differing charges corresponding to image and nonimageareas of an original pattern. In some cases, particularly where theinsulating pattern is only a thin insulating coating on the poorlyconductive layer, the chargeable member may acquire charge in areas C01-responding to the insulating pattern. This effect will disappear afterthe charging master or cylinder has been used a few times.

The term chargeable member used throughout this specification isintended to mean any member on which an electrostatic image pattern maybe formed. Preferably, the chargeable member has good insulatingqualities such as a resistivity in the order of above 10 ohm-centimetersand, preferably, it is uniform in thickness through out. It may behomogeneous throughout, or it may have a layered or laminatedconstruction, provided that at least an outermost layer thereof has thenecessary insulating qualities.

Experiments conducted in carrying out selective charge depositionaccording to this invention have shown that the resistive qualities ofthe poor conducting material, whether it be the covering layer 21 of thecharging cylinder in Figure 1, the background poorly conductive layer 39or 45 of the master layer in Figure 2 or Figure 3, should be within aparticular range. The use of a too highly conductive material willprevent quality charge image formation by distorting selective chargedeposition. The chargeable member, which is an insulating layer, willgenerally have localized spots of high conductivity. When contact ismade with a spot of high conductivity in the chargeable member, theentire charge will travel through the path of least resistance if theresistant qualities of the poorly conductive material are too low. Whenthe resistant qualtities of these areas are low, lateral conductivitytakes place readily, and substantially no charge deposits on thechargeable member in contact with the poorly conductive layer when apoint of high conductivity exists along the line or area of contact.Thus, following selective charge deposition using a poorly conductivelayer of excessively low resistance, the chargeable member will becharged properly in some areas, but in lines, in the case of rollercontact, or entire areas, in the case of the pressure plate type ofcontact illustrated in Figure 3, along which there exists localizedpoints of high conduc tivity in the chargeable member substantially nocharge will be deposited, thereby disrupting charge deposition accordingto this invention. Furthermore, the high current flow created by the lowresistance at the point of high conductivity tends to destroy and breakdown the insulating material of the chargeable member in adjacentsurrounding areas further destroying its subsequent useful' value. Inaddition, the high current flow through a small area of the poorlyconductive layer is often suflicient to cause breakdown in this layeritself. Such breakdown will result in nonuniformity of charge depositionas the poorly conductive layer is used for subsequent cycles. Suchdestructive effects could be eliminated by a suitable resistor in serieswith the power supply, but this would not eliminate the nonuniformity ofcharging. For these reasons, it is desirable to avoid using a poorlyconductive or cover layer made up of excessively conductive material,but in the unlikely event that the chargeable member is altogether freefrom conductive flaws there is no upper limit to the conductivity of thecover layer.

If, on the other hand, too good an insulator is used in areas ofbackground for the covering or poorly conductive layer, charge will bedrawn through the chargeable member and will deposit on the surface ofthe covering or poorly conductive layer rather than on the surface ofthe chargeable member. Excessive resistance in the covering layer willalso unduly increase the time required to effect charge deposition. Theupper limit of insulating characteristics in these areas is thereforeset by the resistance characteristics of the chargeable member. Theresistance characteristics may be defined or determined by the resistivity and the thinness or thickness of the particular layer ofinsulating material included in the chargeable member.

To avoid or minimize the effects of localized breakdown or effectsattributable to spots of high conductivity in the chargeable member, ithas been found that the resistivity of the poorly conductive layershould generally be above ohm-centimeters using as thin a layer ofmaterial in these areas as is possible. The preferred range of practicalresistivity which has been found for areas of background in the coveringor poorly conductive layer is between 10 ohm-centimeters and 10ohm-centimeters. Localized breakdown on the chargeable member does notcause breakdown of the charging system or mechanism using layers ofmaterials for the cover or poorly conductive areas in this range, andselective charge deposition takes place effectively. For purposes ofillustration without any intent to limit this invention, it is suggestedthe various vinyl plastics, as well as black rubber, fabrics, or thelike, may be used as this cover or poorly conductive layer of materialand the chargeable member may comprise, for example, variousphotoconductive insulators such as vitreous selenium; good insulatingplastics such as Mylar or polyethylene, dry insulating paper, or thelike. The photocondutive insulator as the chargeable member is valuablewhen, for example, it is desired to sensitize a xerographic plate with ahalftone pattern or to add a charge pattern to the pattern on the plate.

The insulating characteristics of the insulating pattern area ispreferably in the order of at least 10 times greater than theresistivity elsewhere in the poorly conductive or covering layer.

It is generally desirable to utilize as thin a material as possible inthe background areas. If a point of high conductivity exists in theinsulating layer of the chargeable member, the effect is to create acone of lower potential in the adjacent areas of the master or coveringlayer. The cone tends to distort charging in those areas of thechargeable member beneath areas of the cone in the master coveringlayer. When this layer is thin the cone size is held to a minimum;whereas, when it is thick the cone is larger and the distortion incharge deposition created by the cone is greater on the surface of thechargeable member. Another beneficial feature which flows from a thinnermaster layer is that selective charge deposition takes place within ashorter time period than is necessary if this layer is thicker. Thisfeature allows quicker selective charge deposition. The lower limit ofthinness is determined by finding a layer which will operate withoutdielectric breakdown in itself at a point at which the insulating layerof the chargeable member is broken down. Thus, for example, it has beenfound in charging a chargeable member having a resistivity in the orderof 10 ohm-centimeters and having a thickness of 20 microns using as thepoorly conductive or covering layer a material having a resistivity inthe order of 10 that the thickness in the covering layer should be atleast 4 mils. As the resistivity of the background area of materialbecomes lower a thicker layer is necessary, and as the resistivity ofthis material increases a thinner layer is possible.

Selective charging or charge deposition according to this inventiontakes place to a maximum at which point the charging stops; that is,charge flow according to this invention is self-limiting. When thecharge flow to the surface of the chargeable member reaches a maximumset by the particular configuration of the system or apparatus involvedno further charge flow will take place even though the chargeable memberremains in contact with the poorly conductive layer while backed by theconductive electrode. Thus, once it is determined what particularpotential should be applied because of theparticular arrangementinvolved and because of the charge deposition desired, no fear needexist that damage through breakdown will be caused since chargedeposition will only take place to the maximum set by the potentialapplied to that circuit.

The amount of potential applied to the biased electrode or electrodes isdependent on the particular arrangement being used. For example, in thearrangement in Figure 1, if it is desired to flow 300 volts to thebackground areas of chargeable member 10, conductive core 20 of cylinder15 should be placed at a positive DC. potential in the order of 900volts with respect to backing plate 14. The additional 600 volts, it isbelieved, are necessary because of the gaps present in the arrangement.Thus, in the arrangement of Figure 1 there would be a gap, althoughminute, between the surface of chargeable member 10 and the surface ofcharging cylinder 15 and a gap, although again minute, betweenchargeable member 10 and backing plate 14. Where an additional gap isadded, such as in Figure 2, due to the minute gap between roller 25 andthe charging master 35 carrying insulating pattern 36, a few hundredadditional volts would be necessary for selective charge deposition. Theparticular amount of additional voltage necessary in the particularinstance will depend on such features as the pressure brought to bear bythe rollers or pressure plates, atmospheric conditions, resistivityconditions, and characteristics of the poor conductive material and theinsulating pattern areas, thicknesses involved, and the like. Generally,however, it may be stated that in the various embodiments of thisinvention more than about 600 volts of potential must be applied beforecharge migration will take place through the poor conductive material.

While the present invention as to objects and advantages, as has beendescribed herein, has been carried out in specific embodiments thereof,it is not desired to be limited thereby, but it is intended to cover theinvention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. Apparatus for depositing an electrostatic charge pattern comprising acharging roller having an electrically conductive core and an outercovering layer of material having first areas comprising material havinga resistivity between 10 to 10 ohm-centimeters and second areas having aresistivity at least 10 times greater than that of said first areas, asupport table of electrically conductive material adapted to support achargeable member, means to cause the charging roller to move in rollingcontact with respect to the surface of the chargeable member, and anelectrical power supply to supply D.C. potential between the conductivecore of the charging roller and the support table.

2. The method of depositing an electrostatic charge pattern on achargeable member comprising flowing charge through poor conductivematerial in contact with a chargeable member in areas of background todeposit charge on the surface of the chargeable member while blockingcharge flow in areas of image with insulating material in contact withthe chargeable member.

3. Apparatus for depositing an electrostatic charge pattern on asheet-like chargeable member, comprising a first electrode having anelectrically conductive surface positionable against and in contact witha first surface of the chargeable member, a second electricallyconductive electrode having a surface less conductive than adjacentunderlying portions of said electrode and positionable against and incontact with a second surface of the chargeable member, said surface ofsaid second electrode comprising first areas comprising material havinga resistivity between 10 and 10 ohm centimeters and second areascomprising material having a resistivity at least 10 times greater thanthat of said first areas, an electrical power supply connected to saidelectrodes and adapted to supply a DC. potential therebetween, andpressure means connected to said electrodes to urge them toward eachother and against the chargeable member when supported therebetween.

4. Apparatus according to claim 3 in which the electrical power supplyis adapted to supply a DC. potential of at least about 600 volts.

5. Apparatus for depositing an electrostatic charge pattern on asheet-like chargeable member, comprising a first electrode having anelectrically conductive surface positionable against and in contact witha first surface of the chargeable member, a second electricallyconductive electrode positionable against a second surface of thechargeable member and supporting a covering material less conductivethan said second electrode and having a surface positionable against andin contact with the second surface of the chargeable member, saidsurface of said covering material comprising first areas having aresistivity between 10 and 10 ohm centimeters and second areas having aresistivity at least 10 times higher than the first areas, an electricalpower supply connected to said electrodes and adapted to supply a DC.potential therebetween, and pressure means connected to said electrodesto urge them toward each other and against the chargeable member whensupported therebetween.

6. Apparatus according to claim 5 in which the DC. power supply isadapted to supply a DC. potential of at least about 600 volts. I 1

References Cited in the file of this patent l UNITED STATES PATENTS191,176 Randall May 22, 1877 2,573,881 Walkup et al Nov. 6, 19512,576,047 Schafi'crt Nov. 20, 1951

